I've been using Photoshop for illustration for years. The only thing that I turn to other programs to do is the inking... Photoshop's lines look very jaggy compared to Manga Studio, for example. I like to use Manga Studio for lineart because it's very easy to make vector lines that have no anti-aliased (making it super fast to color with the fill bucket) and are extremely easy to edit. There are tools to smooth the line, enlarge it, thin it, move it... well you try it. It's just really easy and fast and always looks good. I would LOVE to have similar tools on Photoshop, so that I don't have to do the lines somewhere else.